T. Economou and A. Turkevich, Laboratory for Astrophysics and Space Research, University of Chicago, Chicago, IL., USA.
Abstract
The Alpha-Proton-X-ray Spectrometer (APXS) for the Mars Pathfinder
mission is designed to provide a complete and detailed chemical
elemental analysis of Martian soil and rocks near the landing
site.
The APXS instrument is carried on the Pathfinder microrover, which will provide transportation to places of interest on the Martian surface. It consists of a complex sensor head, mounted on a simple, but sophisticated deployment mechanism (ADM) outside the Warm Electronics Box of the microrover (WEB), and the instrument electronics, mounted inside the WEB. The ADM permits to place the instrument sensor head against soil and rock samples in arbitrary positions, ranging from horizontal to vertical, in order to perform in-situ analysis. The possibility to transport the APXS to an arbitrary location, pre-selected on Earth, and to perform in-situ analysis at it, constitutes one of the most exciting aspects of the Pathfinder mission.
The principle of the APXS technique is based on three interaction of alpha particles from a radioisotope source with matter : (a) simple Rutherford backscattering, (b) production of protons from (a,p) reactions on light elements, and (c) generation of characteristic X-rays upon recombination of atomic shell vacancies created by alpha bombardment.
Measurement of the intensities and energy distributions of these three components yields information on the elemental chemical composition of the sample. In terms of sensitivity and selectivity, data are partly redundant and partly complementary: Alpha backscattering is superior for light elements (C, O), while proton emission is mainly sensitive to Na, Mg, Al, Si, S, and X-ray emission is more sensitive to heavier elements (Na to Fe and beyond). A combination of all three measurements enables determination of all elements (with the exception of H) present at concentration levels above typically a fraction of one percent.
OUTLINE
2. Description of the Alpha Proton X-ray Spectrometer Techniques
2.1 Alpha Backscattering (Alpha Mode)
2.2 Proton Emission (Proton Mode)
2.3 X-ray Generation (X-ray Mode)
3. Description of the APXS Instrument
3.1 Sensor Head
3.1.1 Alpha-Proton Detector System
3.1.2 Description of the X-ray sensor head
3.1.3 244Cm alpha radioactive sources
3.2 Electronics for the APXS
3.2.1 Alpha-Proton Analog Board
3.2.2 X-ray Analog Board
3.2.3 A/D and D/A/ Board
3.2.4 The "Gatti" Correction
3.2.5 Microcontroler Board
3.2.6 X-ray Detector Bias and Interface Board
4. The APXS Deployment Mechanism
5. Laboratory Measurements, Calibrations and Environmental Tests
6. Magnetic Target Measurements
7.1 Flight Software for the APX Spectrometer
7.1.1 Command Structure of the APX Spectrometer
7.1.2 Data Structure of the APX Spectrometer
7.1.3 Ground Support Equipment and Software
7.2 Data Analysis
7.2.1 Matrix Effects and Choice of Standards
7.2.2 Least Square Analysis and Programs
7.2.4 X-ray Analysis
1. Introduction
One of the scientific experiments, carried on board the Pathfinder
microrover, will be an Alpha-Proton-X-ray Spectrometer (APXS)
for the determination of the elemental chemical composition of
Martian soil and rocks. Owing to its long standing history, this
instrument could almost be considered a "classic" one,
although it has undergone quite some metamorphosis since its original
conception. This history might be worth telling:
In 1911 Ernest Rutherford, the later Lord Nelson, designed an
experiment to measure the size of atomic nuclei, which today is
considered to be one of the classic key experiments in nuclear
physics: He used alpha particles, emitted from a radioactive source
of polonium, as projectiles and observed their scattering behavior
in the electric field surrounding the nuclei of different target
atoms. From the observed angular and energy distributions of the
scattered alpha particles he estimated the diameter of atomic
nuclei to be of the order of 10-15 m.(Rutherford, 1911).
Some fifty years later Anthony Turkevich et al. of the University
of Chicago employed the same technique (Turkevich, 1961) for the
determination of the elemental chemical composition of Lunar surface
material: Bombardment of the sample with alpha particles from
a radioactive source - this time the transuranium nuclide 242Cm
- and measurement of the energy distribution of these particles
by means of solid state detectors, after they had been scattered
by the target atoms through angles close to 180 (therefore the
term "back-scattered"). They designed instruments, which
flew on three Surveyor missions (Surveyor V, VI and VII) in the
years 1967 and 1968 and yielded the first complete and accurate
chemical analysis of Lunar soils. (Turkevich et al., 1969; Franzgrote
et al., 1970; Patterson et al., 1970). In addition to the measurement
of backscattered alpha particles, these instruments already contained
detectors to measure the energy distribution of protons, generated
by (,p) reactions on certain medium-heavy elements (F, Na, Mg,
Al, Si, S and Cl ), thus enhancing their resolution capability
for these elements. Results of these experiments - including the
determination of an unexpected and comparatively high abundance
of Ti - were later confirmed by laboratory analysis of Lunar samples,
returned to Earth during the Apollo program.(Franzgrote et al.,
1970).
A refined and miniaturized instrument, already including an X-ray
detector (the "Mini-Alpha"), (Economou et al., 1976)
was later proposed for analysis of Martian surface material during
preparation for the Viking missions, but was not selected. Thus
our today's knowledge of the chemical composition of Mars is only
based on the partial results of Viking's X-ray fluorescence spectrometers
(Clark, 1982) and - assuming that they indeed come from
Mars - data from SNC meteorites.(Wänke, 1987).
When Roald Sagdeev, then the director of the Soviet Academy of
Science's Space Research Institute (IKI) in Moscow, was shown
a prototype of this instrument at the occasion of one of his visits
to the University of Chicago, he immediately requested such an
instrument to become part of the science package to be taken to
Mars' satellite Phobos on the Soviet missions with the same name.
Unfortunately, the political climate between the USA and the -
then still - Soviet Union had not become sufficiently temperate
for NASA to consent to such an endeavor. Thus, a similar instrument
had to be designed again in Germany: Dieter Hovestadt et al. (Hovestadt
et al., 1988) of the Max-Planck-Institute for Extraterrestrial
Physics in Garching provided the hardware of the Phobos APX Spectrometers,
employing a telescope of Si detectors for the detection of alpha
particles and protons and a passively cooled Si(Li) detector for
X-ray detection. A group of scientists at the All Union Research
Institute for Atomic Reactors in Dimitrovgrad under the leadership
of Slava Ryadchenko set out to produce sources of 244Cm with the
required specifications and Heinrich Wänke et al. of the
Max-Planck-Institut für Chemie in Mainz undertook to calibrate
the instruments and interpret the data. The Soviets provided a
group of scientists from the Moscow Space Research Institute under
the leadership of Lev Mukhin for scientific cooperation and logistics
support and ultimately Tom Economou of the University of Chicago
was permitted to participate as Co-Investigator.
Unfortunately, both Phobos probes failed to reach their target,
but the instrument was selected for follow-on missions to Mars
itself: First as part of the science payload of the Soviet Mars
Rover "Marsokhod" and then as part of the payload on
both the Small Autonomous Stations and the Penetrators of the
Mars-94 (now Mars-96) mission. But the prerequisite was in either
case a further reduction in size, mass and power consumption of
the instruments, which meant a complete redesign. This work was
attempted as a joint effort between the Max-Planck Institut für
Chemie in Mainz, the University of Chicago, the Space Research
Institute in Moscow and the Research Institute for Atomic Reactors
in Dimitrovgrad. In particular, the University of Chicago took
charge of designing the X-ray part of the combined spectrometer
in pursuit of their research into "room temperature"
X-ray detectors (Iwanczyk et al., 1991; Economou et al., 1992)
on the basis of mercuric iodide. In the meantime more practicable
detectors on the basis of Si-PIN photodiodes have become available
and will be employed for these instruments in the present missions
(Economou et al., 1996).
It was a great honor and pleasure, when the team was invited
to also provide an instrument for the NASA Pathfinder mission.
Except for minor modifications, the Pathfinder instrument is identical
to the ones built for the Small Autonomous Stations of Mars-96.
2. Description of the Alpha Proton X-ray Spectrometer Techniques
The principle of the APXS technique, employed to obtain compositional
information, is based on three interaction of alpha particles
from a radioisotope source with matter: (a) simple Rutherford
backscattering, (b) production of protons from (,p) reactions
on light elements, and (c) generation of characteristic X-rays
upon recombination of atomic shell vacancies created by alpha
bombardment.
Measurement of the intensities and energy distributions of these
three components yields information on the elemental chemical
composition of the sample. In terms of sensitivity and selectivity,
data are partly redundant and partly complementary: Alpha backscattering
is superior for light elements (C, O), while proton emission is
mainly sensitive to F, Na, Mg, Al, Si, S, and X-ray emission is
more sensitive to heavier elements (Na to Fe and beyond). A combination
of all three measurements enables determination of all elements
(with the exception of H) present at concentration levels above
typically a fraction of one percent.
The fact that this technique determines all relevant elements
permits absolute normalization of the results (on a hydrogen
free basis) and makes the analysis insensitive to any variation
in measurement geometry.
2.1 Alpha Backscattering (Alpha Mode)
Elastic collisions between alpha particles and atoms of a target
(sample) material lead to a change in direction and energy of
these particles. This process was first described by Rutherford
in 1911 and is since then referred to as "Rutherford Scattering"
(Rutherford, 1911).
The use of alpha backscattering and related techniques for obtaining
the chemical composition of planetary bodies was described in
detail in the past (Turkevich, 1961; Patterson at al., 1965; Economou
et al. 1970; 1973). The following is a brief review for
easy reference.
The energy E of a scattered alpha-particle, in relation
to its initial energy E0 is a function of the mass A
of the target atom and the scattering angle f :
For a scattering angle of f = 180° ("Backscattering")
this reduces to:
In the case of a thick sample, alpha particles will be scattered
at various depth along their path. Before scattering they will
have lost energy in the sample and the scattered particle will
lose additional energy on its way out of the sample. The resulting
energy distribution is a - generally - flat spectrum, extending
from 0 to a sharp cutoff at a maximum energy determined by E
/ E0 which is characteristic for the scattering element. The
total number of particles registered in the spectrum is a measure
for the number of atoms of the scattering element in the sample,
i.e. its concentration in the sample. These two facts are the
basis for analytical applications of alpha backscattering.
Figure 1 presents in graphical form the dependence of the scattered
energy as a function of scattering angle and element
of mass A. The difference of E / E0 of neighboring elements
becomes largest for scattering angles of 180° and shows only
small variation for angles close to 180°. This is the angular
range, where the highest degree of selectivity can be obtained
by an analytical instrument. As a result of the tradeoff between
selectivity and sensitivity - the number of particles registered
per unit time is linked to the angular range registered in the
detector, i.e. the solid angle observed by the detector - instruments
are usually designed to accept particles scattered through angles
between 150° and 170°.
Based on a model of pure Coulomb-interaction, Rutherford also
derived an expression for the cross section s (in cm2 per atom
and steradian) of this scattering process as a function of the
energy E (in MeV) of the a-particle, the atomic charge
(atomic number) Z of the target atom and the scattering
angle f:
For a scattering angle of 180° and an energy E of
5.8 MeV, this model predicts values for s between 5*10-27 [cm2/at/sterad]
for carbon (Z=6) and 1*10-25 [cm2/at/sterad] for iron (Z=26).
While these values are valid for heavier elements (Z>20), cross
sections for light elements tend to be higher by up to a factor
of 100 due to nuclear interactions. This is in particular true
for carbon and oxygen, where resonance scattering processes play
an important role.
2.2 Proton Emission (Proton Mode)
Another process important for analytical applications is the
nuclear (a,p) reaction: Alpha particles merge with the target
nucleus, followed by the emission of a proton and - in some cases
- gamma radiation. This process is characterized by the Q-value,
i.e. the difference in binding energy of the alpha-particle and
the target nucleus on the one side and of the proton and the product
nucleus on the other side. This process is energetically possible,
when the kinetic energy of the incoming alpha-particle E exceeds
the difference in binding energy Q; the excess energy is transferred
to the kinetic energy of the proton Ep and the energy of
an associated gamma transition Eg:
Ep + Eg = E + Q
This process is of particular interest in the case of the light
rock-forming elements Na, Mg, Al and Si, where Q-values
range between -2 MeV and +2 MeV and the reaction cross sections
for alpha-particles of 5 to 6 MeV are not too small. This is due
to the fact that alpha-particles have to penetrate the Coulomb
barrier of the nucleus, before the nuclear reaction can take place,
and this is determined by the nuclear charge of the target nucleus.
Table 1 shows the Q-values for some selected target nuclei.
Table 1: Q-values for (a,p) Reactions
2.3 X-ray Generation (X-ray Mode)
The alpha particles from the radiation sources used in the alpha
and proton modes are also used as a very efficient excitation
source for production of characteristic X-rays from the sample
material. Actually, charged particle excitation is preferred to
any other kind of excitation since it produces the best signal-to-noise
ratio due to absence of any Compton scattering. This advantage
significantly improves the performance of the instrument. The
addition of a small X-ray detector and only some additional electronics
results in a significant extension of the accuracy and sensitivity
of the Alpha - Proton instrument, particularly for the heavier,
less abundant elements.
The analytical information in the X-ray mode comes from the characteristic
X-rays that are emitted when the low electron orbit vacancies
(in K and L shells) produced by bombardment of atoms by alpha
particles are filled by electrons from higher orbits. The alpha
particle sources can excite characteristic X-rays in a sample
in two ways. First, the interaction of the alpha particles with
the electronic cloud of an atom has a probability of producing
a vacuum in the K electronic shell of the target. This produces
characteristic X-rays with a cross section roughly varying as
E4/Z12. Second, alpha radioactive sources such as 244Cm are also
strong emitters of L X-rays themselves. These have energies of
~15 to 22 keV, and can produce characteristic X-rays in measured
sample. The sensitivity for a particular element or group of elements
can be additionally enhanced by inclusion of auxiliary excitation
sources in addition to the primary alpha sources. (Economou et
al., 1976).
In addition, the X-ray mode of the APXS is very helpful in another
way. While alpha mode has very good resolution for separating
the light elements, it starts to have problems in separating the
neighboring elements above about the element silicon. The opposite
is true for the X-ray mode: it has its best resolution exactly
where the alpha mode has the worst resolution. Figure 2 shows
schematically the resolution power and the ability to separate
the adjacent elements for the two modes.
3. Description of the Alpha Proton X-ray Spectrometer
The APXS consists of two parts: The sensor head and the electronics
box. The sensor head (dimensions 52 x 71 x 35 mm) is mounted on
a deployment mechanism outside the Pathfinder microrover's Warm
Electronics Box (WEB). The electronics box (dimensions 70 x 80
x 65 mm) is contained inside the WEB. The sensor head is connected
to the electronics box via four coaxial cables (alpha-, proton-
and X-ray signals; X-ray bias voltage) and six single wires (AWG
28, power for the X-ray preamplifier and the shutter motor; temperature
sensor). Figure 3 is a photograph of the flight APXS showing the
sensor head on the left and the electronics box on the right with
their associated cables. Table 5 lists the mechanical and electrical
specifications of the instrument.
Table 5: Main characteristics of the APXS.
Target Isotopic Abundance (%) Reaction Q-Value (MeV)
6Li 7.4 6Li (a,p) 9Be -2.1
7Li 92.6 7Li (a,p) 10Be -2.6
9Be 100 9Be (a,p) 12Be -6.9
10B 19.6 10B (a,p) 13C +4.1
11B 80.4 11B (a,p) 14C +0.8
12C 98.6 12C (a,p) 15N -5.0
13C 1.1 13C (a,p) 16N -7.4
14N 99.6 14N (a,p) 17O -1.2
16O 99.7 16O (a,p) 19F -8.1
19F 100 19F (a,p) 22Ne +1.7
23Na 100 23Na (a,p) 26Mg +1.8
24Mg 78.7 24Mg (a,p) 27Al -1.6
25Mg 10.1 25Mg (a,p) 28Al -1.2
26Mg 11.2 26Mg (a,p) 29Al -2.9
27Al 100 27Al (a,p) 30Si +2.4
28Si 92.2 28Si (a,p) 31P -1.9
32S 95.0 32S (a,p) 35Cl -1.9
Weight 570 g
Power Consumption 340 mwatts
Voltages 6.0 to 15 DCV
Volume 1. Electronics box 375 cm3
Sensor Head ~85 cm3
Radioactive Sources 50 mCi of Cm-244
Operation Time ~600 minutes/sample
Required Data Volume 16 k/sample analysis
Communication Protocol RS-232, TTL level, 9600 b/s
3.1 APXS Sensor Head
The sensor head contains nine 244Cm sources in a ring-type geometry and three detectors for the measurement of the three components: A telescope of two Si-detectors for the measurement of alpha-particles and protons and a Si-PIN X-ray detector with its preamplifier.
Figure 4 shows the geometrical arrangements of all components of the sensor head: Sources are contained in their own holder and are protected by a motor-driven shutter of 0.2 mm thick stainless steel blades and very thin (typically 200 nm thick) foils of alumina and VYNS. Collimators, delineating the area to be analyzed, are placed in front of the detectors, rather than in front of the sources, as this yields a more compact design. These collimators have been designed for a nominal working distance (distance between sample surface and collimator front face) of 4 cm. This distance is, however, not very critical and may in a real situation vary by as much as ±0.5 cm.
Figure 5 is a photograph, showing the APXS sensor head, mounted with the deployment mechanism on the back of the rover. A color camera mounted on the back of the microrover is visible on the right side of the APXS. This camera will provide close up images of all samples analyzed by the APXS.
3.1.1 Alpha-Proton Detector System:
The maximum energy in the backscatter spectrum of 244Cm is the emission energy of 244Cm , i.e. 5.80 MeV. A Si-detector of 35 mm thickness (D1) will completely stop alpha particles of 6.5 MeV, which means that there is a sufficient reserve for range straggling and partial channeling such that no backscattered alpha particle will penetrate this detector. On the other hand, this detector will become transparent for protons of an energy greater than 1.6 MeV. A second detector (D2) behind the 35 mm detector, thick enough to completely stop protons of energies up to 6 MeV (> 320 mm) will register these protons and the sum of the signals from both detectors will correspond to the total proton energy. With the help of threshold discriminators and a coincidence logic, events caused by alpha particles can be distinguished from events caused by protons and thus alpha spectra and proton spectra can be recorded separately. Figure 6a shows the energy deposited in the thin detector D1 and the thick detector D2 by protons, emitted from the sample. Using threshold settings are 0.4 MeV for both detectors, proton events can be distinguished from alpha events, if the proton energy lies between about 1.8 MeV and 6 MeV. Protons with an energy of less than 1.8 MeV are registered as alpha events. In practice, this is not critical, because (I) the significant part of the proton spectra is contained in the energy range above ~ 2 MeV and (ii) proton-events occur at a significantly lower rate than alpha events such that the alpha spectra are not noticeably disturbed by the presence of low energy protons.
The thick detector D2 plays a second important role as an active anticoincidence shield against cosmic ray protons. A proper choice of its thickness permits to suppress such events in the data evaluation, partly based on coincidence conditions and partly on the amplitude of the signals. Figure 6b shows the energies deposited by cosmic ray protons first striking a detector of 700 mm thickness (D2) and subsequently a detector of 35 mm thickness (D1). As can be seen from the figure, protons with an energy of less than ~ 9.8 MeV are completely stopped in the thick detector. Protons with energies above 9.8 MeV will also deposit energy in the thin detector. However, either the sum of the signals from both detectors is larger than the range of interest (6 MeV) or the signal in the thin detector is too small to exceed the threshold of the discriminator (0.4 MeV). A signal, only recorded by the thick detector, is discarded as an unwanted cosmic ray background event.
The above described detector arrangement ("telescope") requires that detector D1 is fully depleted, i.e. its "active" thickness is essentially identical with its physical thickness. The rear detector D2 must have an active thickness of at least 700 mm. Full depletion is not mandatory for this detector. However, the dead layer on the side facing detector D1, should be as thin as possible. The detectors must be mounted as close together as possible to minimize the solid angle, under which particles can arrive at D1 without passing through D2.
3.1.2 Description of the X-ray sensor head
The X-ray mode of the APXS uses solid state X-ray detectors, which are the result of the latest technology development. They operate at or slightly below room temperature. The elimination of the cryogenic resources and any associated plumbing related to such a system enables an exceptional degree of miniaturization of the APXS instrument. Figure 7 shows schematically the arrangement of the X-ray detecting system and its components. It consist of the silicon PIN photodiode X-ray detector mounted on a beryllia substrate on top of the cold side of a Peltier cooler, the front end of a charge sensitive preamplifier, and, a temperature sensor. All these components are enclosed in a hermetically sealed metal container the size of a TO-8 can, filled with inert gas (made by AMPTEK corporation). A thin (8 micron) beryllium window in the front enables entry even for very low energy X-rays into the detector system. An important part of the system is a tungsten ("heavy met") collimator inside the hermetically sealed container. Its purpose is twofold: It collimates the X-ray detector, so it analyzes the same sample area as the alpha and proton detectors, and, at the same time it shields the detector from the X-rays and other gamma-rays coming directly from the alpha radioactive sources that due to tight geometry are very close to the X-ray detector. The small Peltier cooler, consuming about one watt of electrical power, is very convenient during laboratory testing, final integration and environmental testing on board the spacecraft. For this purpose it is powered from a D-size 1.5 V battery that lasts for about 6 hours of continuous operation. It will not be needed during the operations on the surface of Mars.
The output from the fist stage of the preamplifier is fed into the charge sensitive preamplifier sitting on the top of the sensor head. Since the preamplifier is not inside the temperature controlled compartment, its temperature will be in equilibrium with Martian ambient temperature in the range of -100 C to +10º C. The preamplifier was designed for proper operation at the Martian temperature range. It was tested and operated down to a temperature of -120º C.
3.1.3 244Cm Alpha Radioactive Sources
The APXS needs for its operation in alpha, proton and X-ray modes a beam of alpha particles with high intensity and low energy spread: Intensity of the beam determines the total measurement time needed to obtain data with the necessary statistical accuracy; its energy spread directly determines the resolving capability of the alpha mode. For space applications, such a beam is most conveniently obtained from a radioactive source. In this case, however, intensity and energy spread are linked together and a suitable compromise has to be met: Given a finite source area, the intensity is determined by the amount of source material, i.e. its thickness, and its specific activity (determined by its half-life). On the other hand, alpha particles, emitted from within a thick source, loose energy on their path through the source material. Thus, thick sources exhibit an inherent energy spread. It is therefore desirable to use radioisotopes with a short half-life, i.e. with high specific activity. In practice, the time between preparation of the source and its use sets a limit to the minimum useful half-life. 244Cm with a half-life of 18.1 years was chosen as suitable for applications on Mars.
Another important factor, determining the resulting energy spread of a source, is the chemical composition of the source material: Ideally one would use the source material in elemental form. In the case of curium such sources, however, tend to be chemically unstable and to rapidly deteriorate. In the past ten years an extensive research program has been undertaken by S. Ryadchenko et. al. at the Research Institute for Atomic Reactors in Dimitrovgrad, Russia, in search of suitable chemical forms for high quality sources. Intermetallic compounds with various metals (Pt, Ir, Rh, Pd) have been investigated and yielded promising results. More recent work concentrated on the formation of curium silicides on semiconductor grade silicon. This technology has yielded the best results so far and sources for the current Martian projects are manufactured by this technique. Table 6 shows the characteristics of these sources. Their energy spread is described by full width at half maximum (FWHM), 10% of maximum (FW0.1M) and 1% of maximum (FW0.01M).
Table 6: Characteristics of the 244Cm alpha radioactive source used for the Mars Pathfinder APXS instrument.
Source Isotope: 244Cm Number of Sources 9 Total Intensity 50 millicuries (1.85*109 Becq.) Ea 5.807 MeV T 18.1 years FWHM 2.3 % FW0.1M 3.5% FW0.01M 10.0%
3.2 Electronics for the APX Spectrometer
Basically, the electronics of the APXS consists of three independent analog channels for each of its modes: alpha, proton and the X-ray channel, and the digital electronics to condition signals and handle the data produced by the instrument. All of the APXS electronics, except for the X-ray preamplifier, is inside the rover warm electronic box (WEB), the temperature of which will be controlled within a range of -50 ºC to +50 ºC, using power from batteries and auxiliary thermal energy from radioactive plutonium heaters. The temperature of the sensor head, including the X-ray preamplifier will be at Martian ambient temperature, expected to be in the range of -100 ºC to +10 ºC.
Figure 8 shows the block diagram of the electronics system for the APX Spectrometer. Six individual building blocks are indicated by dashed lines. They consist of the sensor head with alpha, proton and X-ray detectors (with the X-ray preamplifier) and five printed circuit boards with
(1) the analog section for the alpha- and proton detector, up to and including peak
detector/stretcher;
(2) the analog section for the X-ray detector, up to and including peak detector/stretcher;
(3) serial A/D and D/A converters, voltage reference source and buffer amplifiers;
(4) microcontroler with program PROM, data RAM, serial I/O, a watchdog/power
monitor circuit and a backup battery;
(5) the voltage converter for the X-ray detector bias, power line filters and the interface
connectors (not shown).
Before the boards are described in detail, some words should be said about the general concept: The system performs the tasks of
- amplifying and conditioning individual pulse signals from three detectors,
- measuring their amplitude, which in turn is a measure of the energy, a charge particle or
an X-ray photon has deposited in the detector, and
- counting the number of events in 256 pulse height intervals per detector.
Measurement of the pulse amplitude and event counting is performed under control of a microcontroler, which also communicates with a host system via an asynchronous serial interface (RS 232 protocol, TTL levels), from which it receives commands for the operation of the system and to which accumulated data are transmitted.
The rather stringent requirements for instruments on the Small Autonomous Stations of the Mars-94 program with respect to low power consumption and small mass and size could only be met using state of the art components, most of which have not been qualified for space applications. All parts used are commercially available in MIL versions, i.e. specified for the temperature range of -55 C to +125 C and packaged in either hermetic ceramic dual-in-line packages or hermetic metal cans. Some are even available in MIL/883 versions, the others had to be individually qualified. Estimates of the radiation environment on Mars led to rather moderate requirements with respect to radiation hardness. Nevertheless, for critical components like the microcontroler and its program memory, radiation-tolerant versions have been used.
The electronic system was built on five printed circuit boards of dimensions 70 x 80 mm and is housed inside a metal box of dimensions 71 x 81 x 65 mm. The boards are designed for active components in standard ceramic dual-in-line components. Passive components are used both in conventional wired and surface-mount versions.
As the system will have to operate over a rather wide temperature range (-50C to + 50C) and building analog circuits with sufficiently small temperature coefficients proved very difficult (the main reason for a change of the signal amplitude versus temperature is the temperature coefficient of the feedback capacitor in the charge sensitive preamplifier), a digital compensation concept has been adopted, which uses digital-to-analog converters (D/As) to control the gain of the analog amplifiers. The temperature of the critical parts (preamplifiers) is measured in regular intervals (typically several minutes) and the D/As are reprogrammed with values from calibration tables. In a similar way D/As are used to compensate (temperature dependent) zero point offset errors and to define threshold voltages of critical discriminators.
Measurement of the pulse amplitudes and signals from the temperature sensors (AD 590 by Analog Devices) is accomplished by means of an analog-to-digital converter (A/D, LTC 1094 by Linear Technology) with 10 bit resolution, 8 multiplexed inputs and a synchronous serial interface to the microcontroler. This converter has a power consumption of 7.5 mW.
Use of D/A converters and an A/D converter with synchronous serial interfaces greatly simplifies interfacing to the microcontroler and provides for better isolation from the microcontroler bus.
3.2.1 Alpha-Proton Analog Board
The detector signals are fed to charge sensitive preamplifiers, modified for detectors with large capacitance (A 225 H by Amptec). Their outputs are connected to the reference inputs of multiplying current output D/A's which, in combination with external low power operational amplifiers (MAX 403 by Maxim) form digitally gain-controlled main amplifiers. From these amplifiers the signals are brought to individual comparators (MAX 909 by Maxim), whose thresholds are digitally set to the equivalent of 400 keV energy deposited in either the alpha or the proton detector. Signals larger than this threshold trigger the logic circuit (54HC74, 54HC08), which generates control signals for the peak detector/stretcher, a timing signal for the coincidence condition and an interrupt signal for the microcontroler. This circuit is reset by the microcontroler after the pulse amplitude has been measured and the event has been registered in its appropriate memory location.
The same signals are also brought to a summing amplifier (MAX 403) and the peak detector/stretcher circuit (MAX 403, IH 5141 by Maxim; OP 43 by Analog Devices/PMI). The output of this circuit provides a signal corresponding to the maximum amplitude of the sum of the detector pulses to the A/D converter. This signal remains unchanged until the circuit is reset by the microcontroler: The CMOS switch IH-5141 serves to disconnect input from output during analysis and to discharge the hold capacitor after analysis and during normal acquisition.
The board also contains a low power (3 mW) +5 V to + 25 V converter (40106B Schmitt trigger oscillator with voltage multiplier cascade), which generates the bias voltage for the alpha and proton detectors.
3.2.2 X-ray Analog Board
The signal from a low noise charge sensitive preamplifier (hybridized by AMPTEK Inc.) is directly fed into a low power spectroscopy type amplifier specifically designed to handle the positive amplitude pulse from the Si-PIN X-ray detectors. The two stage amplifier based on dual HA5112 and HA5152 OP amps delivers an amplitude of 2.5 V for a 15 keV X-ray. The amplifier was optimized in terms speed, power consumption and noise performance by selecting a shaping time of about 23 ms. The gaussian signal after the proper amplification is then fed into the peak detector/sample hold (based around CD4066A) that conditions it for the ADC contained in the alpha A/D board. A logical flag is also generated at the same time to signal the microcontroller to process the X-ray event. Microcontroller, after finishing processing an X-ray event sends a reset signal enabling analysis of the next event.
The interfacing of the X-ray analog board with the A-P boards is via pins at the end of the PC board that directly plugs to any other board below it. The same is true for all other APXS boards. This unique arrangement enables for easy assembly, testing and interchanging of boards between different APXS units. All components of the X-ray analog board were selected and tested to comply with MIL/883 specifications.
3.2.3 A/D and D/A Board
This board uses five serial "daisy chain" D/A converters (DAC 8143 by Analog Devices/PMI) for the adjustment of X-ray gain (the D/A for alpha and proton gain are contained in the respective signal chains, see above), offset and "Gatti" corrections (sum of alpha and proton, X-ray; for "Gatti" see below) and threshold settings (alpha, proton), and a serial 8 input A/D converter (LTC-1094). Also contained on this board, but not shown in the block diagram, are a +2.5 V reference source (AD 580 by Analog Devices) and a 3 to 8 line decoder (54 HC 138), which generates chip enable and other signals used by the system.
Gain control for the X-ray signal is achieved by feeding the signal from the pulse stretcher through a D/A converter, i.e. using the D/A as a digitally controlled attenuator. Offset and "Gatti" correction is done after the pulse stretcher. This has the advantage that offset errors of the stretcher circuit can be taken care of and that the amplifiers connected to the D/As can be slow and of low power consumption.
3.2.4 The "Gatti" Correction
The differential nonlinearity (DNL) of successive approximation A/D converters is usually quoted by the manufacturer to be less than ± 1/2 LSB. In practice this figure may be smaller, but for design purposes ± 1/2 LSB has to be assumed. Using a converter with 10 bit resolution, the expected DNL, referred to 256 channels (8 bits) will be ± 12.5 %, i.e. channel width can vary by as much as ± 12.5 %.
On the other hand, the variation of channel width of a good multichannel analyzer should be less than ± 1 %. This means that for a 256 channel spectrum (8 bits) a converter is needed, for which 1/2 LSB corresponds to 1 % of 1/256 of full scale (1/25600), i.e. a converter with an equivalent resolution of 14 bits (1/2 LSB = 1/32768).
To improve the DNL obtainable with converters of lower resolution (that translates to low power), a method is employed, which was originally suggested by Gatti et al. in 1963 (Gatti 1963): An analog signal, generated by a precise D/A converter and corresponding to n times the mean channel width in the spectrum (n = 0, 1, ...., m), is added in the analog section before the A/D converter to the signal to be digitized and subtracted digitally from the conversion result. For periodic signals the number n is generated at random; for signals occurring at random, a systematic adjustment, e.g. by a counter that is incremented after each measurement cycle (up to a value m and then reset to zero), is permissible. The effect of this procedure is that signals of a certain amplitude (corresponding to channel x) are subsequently digitized into channels x, x+1, x+2, ....x+m. After subtraction of 0, 1, 2, ...., m they are assigned to channel x, where they belong. The variation of channel width, however, is now the mean of the variations of the m+1 adjacent channels x, x+1, x+2, ....., x+m. In general this procedure reduces the variation by about a factor of m. In our case m = 16 is used, thus reducing the DNL to less than the required 1 %.
3.2.5 Microcontroler Board
This board contains the 8 bit microcontroler (80C31), 2K x 8 bit program memory (fuse link PROM HM 6617 by Harris) and 32K x 8 bit data RAM (MSM 832 by Hybrid). A watchdog circuit/power monitor (MAX 695 by Maxim) generates reset signals on power up and when a program error occurs and disables the data RAM and connects its power supply to a backup battery, when system voltage becomes low. Finally, the board contains Schmitt trigger buffers for the serial lines (with an input protection network for non-TTL levels).
Eight lines of Port1 and four lines of Port3 are used for interfacing with the rest of the system. System clock operates at a frequency of 7.3728 Mhz.
The microcontroler operates in an interrupt driven mode, i.e. after initialization it enters its power saving idle mode and resumes normal operation only in response to internal (Timer, Serial I/O) or external interrupts to perform one of these tasks:
- respond to commands received through the Serial Interface,
- respond to signals from the analog electronics to perform signal amplitude
analysis and multichannel storage,
- respond to Timer signals to periodically increment a counter for the measuring
time, combined with periodic temperature measurements and the
associated readjustment of D/A-settings.
3.2.6 X-ray Detector Bias and Interface Board
The Si-PIN X-ray detector from the AMPTEK corporation needs for its proper operation to be biased to +80V-100 V DC. It is a paramount requirement that the bias supply is stable and well filtered in order to avoid introducing any noise to the X-ray system.
Such a bias supply is being generated in the X-ray detector bias board from +7.5V line using CD40106B Schmitt trigger oscillator with multiple stage voltage multiplier cascade. Several jumpers are provided for selection of bias supply between 80 and 100 V.
Also, built-in in this board is a solid state power switch ( HI1-302 by Harris) that can turn off the power to the X-ray system when it is not needed. The in-rush current for the X-ray system is limited to some extent by incorporation of 2 mH inductors on each power line before the solid state switches. The bias power supply board provides most of the filtering of the power for the X-ray system and some for the A-P system. Additional filtering was introduced in this board to reduce the noise problem encountered during the interfacing with the rover electronics. This board handles all the cables to the sensor head and to the rover electronics.
4. The APXS Deployment Mechanism
One of the most exciting aspects of the Mars Pathfinder APXS experiment is the way it will be deployed to analyze Martian surface soil and rock samples. While usually, as for example on the Russian Mars-96 mission, the APXS instrument is deployed after the landing, and therefore it will analyze whatever single sample happens to be under the instrument, the APXS on the Mars Pathfinder is mounted on one end of rover that will provide it with an unlimited mobility around the lander site and therefore it would enable it to analyze multiple soil and rock samples. The deployment mechanism was designed at JPL as part of the rover in a such a way that the APXS can be deployed vertically to the ground or horizontally against any rock that looks interesting and was pre-selected by the lander or rover images. The deployment mechanism is a very ingenious device, being operated with only one motor, but providing three axes compliance to the analyzed sample shape. Contact switches and spring coils makes the design simple and dependable. Photographs on the figures 9a and 9b show the deployment mechanism placing the APXS vertically for soil analysis and horizontally against a rock.
The following are some of the requirements that the ADM was design to:
1. +45 to -100 C non-operating 2. +25 to -100 C operating
5. Laboratory Measurements, Calibrations and Environmental Tests
The following APXS instruments were built for the Pathfinder mission:
breadboard
engineering model
laboratory unit
flight unit
second flight sensor head
The breadboard and the engineering units were used mainly for mechanical and electrical interfacing with the rover and during all environmental and qualification testing. The laboratory unit which is identical to the flight unit will be used in the laboratory to derive the elemental libraries and to establish the accuracy and the detection limits of the Mars Pathfinder APXS instrument. For that purpose a second 244Cm alpha radioactive source set which is identical to the flight source will be used with the laboratory unit. The same source set will also be used to cross calibrate the second flight sensor head with the laboratory instrument. This second sensor head, together now with the flight 244Cm alpha sources, will replace the one on the flight unit and it will become the flight sensor head.
The detailed calibrations obtained by the laboratory instrument will then be used to analyze the data obtained by the APXS during measurements of the Martian surface samples. The calibrations of the flight sensor head with the flight alpha sources and the cross calibration with the laboratory sources will be performed before delivery, while the detailed laboratory calibrations will be done during the period after the delivery and prior to the landing.
From preliminary laboratory measurements it was established that the APXS is performing as expected and all parameters (gain, thresholds, temperature corrections etc.) were set properly. Figure 10a and Figure 10b show typical alpha and proton spectra of an igneous rock obtained with the APXS instrument. Similarly, Figure 11a and Figure 11bshow the quality of data expected to accomplish during the mission with the X-ray mode of the APXS. This is the X-ray spectrum of Allende meteorite obtained by the laboratory instrument using 244Cm flight quality alpha excitation sources. The resolution of the Si PIN based X-ray detecting system is now even better than the previous version based on Mercuric Iodide X-ray detectors: It is good enough to separate the K lines from almost all elements and for the heavier elements even the K lines. There were other advantages that persuaded us to switch from HgI2 to Si PIN photodiode X-ray detectors: Silicon, besides being a much easier material to handle and procure for space applications, produces significantly better signal-to-noise ratio, especially in the 1-10 keV X-ray region. HgI2 detectors, due to their high Z composition, are very effective in registering the unwanted background producing high energy X-rays and gamma rays from the 244Cm radioactive sources, while silicon based detectors are transparent to these energies. It can be clearly seen by comparing the Fig. 11a and 11b of Allende X-ray spectra, taken under identical conditions, that the signal-to-noise ratio for silicon detectors is more than 10 times better, than that for HgI2 X-ray detectors. Low abundance elements Ti, Cr, Mn are now clearly visible and even the 300 ppm potassium line can be identified with Si detectors, while they were buried in the background noise with HgI2 detectors.
All the units of the APXS for the Pathfinder mission were tested to comply with vibration, shock and thermal specifications as defined by the Russian Mars-96 project. These are generally more stringent than Pathfinder project specifications. For example the Pathfinder specifications call for shock testing only to 70 g level while Mars-96 requires to qualify all instruments to withstand a 200 g shock level.
6. Magnetic Target Measurements
During operations on the surface of Mars, the APXS will analyze many samples of Martian soil and rocks. It is, however, also anticipated that the APXS will analyze several magnetic targets mounted on the ramp of the lander. These targets are provided by the Niels Bohr Institute of Copenhagen University in Denmark and their purpose is to provide information about the magnetic properties of the Martian surface material. This experiment is similar to the one on the Viking missions, but with more balanced magnetic targets to cover the expected range of magnetic susceptibility of Martian material. It is expected that wind blown material in time will be collected on the surface of the magnetic targets of different magnetic strengths. By analyzing the collected material with the APXS it will be possible to determine if there is any preferential separation of the collected material according to the magnetic properties of the material.
The spectra of the magnetic targets obtained during the Martian operations will be compared with the laboratory spectra obtained during the calibrations of the APXS instrument. From these measurements, it is expected to determine some of the iron mineralogy of the Martins surface material. (see Knudsen et al).
7.1 Flight Software for the APX Spectrometer
The general concept for the flight software is the following:
After a power-on reset, the APXS microcontroller performs initialization
and then enters a low power "sleep" (idle) mode. From
this mode it is "woken up" by external interrupts to
perform various tasks in interrupt - service routines. These tasks
are:
- response to commands received through the Serial Interface,
- response to Timer0 signals to periodically increment
a counter for
the measuring time, combined with periodic temperature
measurements
and the associated readjustment of D/A-settings,
- response to signals from the analog electronics to perform
signal
amplitude analysis and multichannel storage.
The program consists of 5 independent blocks:
Communication between these blocks is accomplished by means of
flag-bits and RAM cells in the external, battery-buffered data-RAM.
In the same way information is communicated between subsequent
measurement cycles (when power is switched off between cycles).
7.1.1 Command Structure of the APX Spectrometer
The system will respond to the following commands:
"CYCLE_START" (0E9H) This command creates a backup
of the cumulative
spectra and erases the current and cumulative
spectra..
"MEAS_START" (0D1H) This command starts collection
of data in the current
working area.
"MEAS_STOP" (062H) This command stops data collection
and marks
spectra in the current working area as valid.
"RESET" (06DH) This command simulates a power-on-reset
of the
processor.
"TX_START" (043H) This command prepares transmission
of data.
Subsequently
"TX_BYTE" (0E4H) is sent for each byte to be transmitted.
A minimum
of 2048 "TX_BYTE"s read the content of the most
recent cumulative spectra; additional "TX_BYTE"s
read backups.
Data transmission is then terminated by the command
"TX_STOP" (0C5H).
"OPEN_SHUTTER" (0D1H) This command opens the shutter
in front of the
sources.
"CLOSE_SHUTTER"(021H) This command closes the shutter
in front of the
sources.
7.1.2 Data Structure in the APX Spectrometer:
During each measurement session four spectra of 256 channels
are accumulated. Each channel consists of two bytes, organized
as an event counter (each channel can contain a maximum of 65535
counts; the channel number corresponds to the amplitude of the
event, i.e. the energy of the registered particle/photon), thus
each spectrum consists of 512 bytes and the whole set of four
spectra of 2048 bytes. The first spectrum (AL_C) contains events,
registered by the alpha detector only. The second spectrum (PR_C)
contains events registered simultaneously by the alpha and the
proton detector (the amplitude being the sum of the amplitude
of both signals). The third spectrum (XR_C) contains events registered
by the X-ray detector and the fourth spectrum (BG_C) contains
events registered by the proton detector only (essentially cosmic
ray background events).
In order to yield spectra with sufficient statistical accuracy,
the typical total accumulation time per sample should not be less
than 10 hours. It was anticipated that this can not be guaranteed
in one single, uninterrupted session. Therefore the following
philosophy was adopted: Data from each measurement session are
stored in four spectra in the current working area (CWA). These
spectra contain one marker, set by the "MEAS_START"
command and a second marker, set by the "MEAS_STOP"
command. After system start (power-on reset) these markers are
checked to establish validity of the spectra and only valid spectra
are added to a second set of four spectra, the so called cumulative
spectra (AL_L, PR_L, XR_L, BG_L; L stands for "last cumulative",
see below). After this validity check (and transfer to the cumulative
spectra) the CWA is cleared for a new measurement session.
Upon transmission of data (initiated by the "TX_START"
command), data from these cumulative spectra are read (2048 bytes).
After reading these 2048 bytes and terminating transmission with
a "TX_STOP" command, these data are copied to the next
2048 bytes in memory as a backup. Physically, these data are located
in the APXS data RAM at the following addresses:
In order to clear memory for the measurement of a new sample,
the command "CYCLE_START" clears both the CWA and the
cumulative sum of sessions 1 to n. Before clearing, however, a
backup of the cumulative sum is performed. This has the following
consequences: (1) repeated issue of the "CYCLE_START""command
will ultimately (after ten times) clear all backup copies and
fill the memory with zeros. (2) Repeated reading of more than
2048 bytes without new accumulation of data will fill all backup
copies with identical spectra (the content of the cumulative,
1 to n).
7.1.3 Ground Support Equipment and Software
The APXS Ground Support Equipment (GSE) consists of a laptop
computer, a 7.5 V power supply, and the appropriate power and
data cables.
A simple program, GSE_PFR.EXE, permits to communicate with the
APX Spectrometer during testing, integration with the rover and
also during the calibrations and laboratory measurements. All
the APXS commands used with the flight software and several non-flight
commands are available through function keys or from the laptop
keyboard. The program also contains routines for displaying, plotting
and printing all the APXS energy spectra.
7.2 Data Analysis
Measurements with the APX Spectrometer yield three data sets:
(1) a spectrum of backscattered alpha particles,
(2) a spectrum of protons generated by (,p) processes and
(3) a spectrum of characteristic X-rays emitted from the sample
upon excitation
with alpha particles and X-rays.
Within certain constraints (see matrix effects below) all three
spectra can be considered linear superpositions of spectra of
all elements present in the sample, multiplied with an appropriate
scaling factor linked to their abundance in the sample:
The alpha spectrum (1) is the sum of back-scatter spectra of
all elements with atomic mass A> 4 (He, this is due to the
physics of the back-scattering process); the proton spectrum (2)
is the sum of proton spectra emitted by elements, for which (,p)
reactions take place (mainly Na, Mg, Al, Si and S) and the X-ray
spectrum (3) is the sum of X-ray spectra emitted by all elements
heavier than Na (this from technical reasons associated with
the type of detector used).
In principle, abundance figures can be derived from each of the
three types of spectra; in practice, a combination of the results
is required to overcome certain limitations of each approach and
improve the accuracy of the results:
In the alpha spectra, the low resolving power of the instrument
for elements heavier than Mg and the statistical counting errors
of the data do not permit accurate distinction between the major
rock forming elements Mg, Al and Si. On the other hand, these
are the main elements contributing to the proton spectra.
The X-ray spectra provide information on elements heavier than
Na, but matrix effects (absorption and secondary fluorescence)
play a more important role, than in the case of alpha and proton
spectra.
The approach taken is therefore an iterative one: In a first
step data from the alpha and proton spectra are combined and the
complex sample spectrum is decomposed into its individual components,
using a least square fitting procedure with a library of standard
spectra, and applying appropriate corrections for matrix effects.
As the alpha-proton spectra contain information about all elements
heavier than He, neglecting the lightest elements, the results
can be normalized to add up to 100 %. Therefore, an accurate knowledge
of the measurement geometry (and of the measurement duration)
is not required.
In a second step the X-ray spectra are analyzed, using a library
of standard spectra and the results from the first step for matrix
corrections. This step yields improved data for the ratios of
the elements Na through Ni, which are used in a second least squares
fit of the alpha-proton spectra.
7.2.1 Matrix effects and choice of standards
In the case of alpha and proton spectra the total composition
of the sample influences the stopping power for both alpha particles
and protons, but as long as the stopping power functions of different
elements show the same general dependence on energy, the shape
of the spectra of different elements does not change and the composite
spectra are truly linear combinations of the spectra of individual
elements. Concentration numbers can then be derived from the decomposed
spectra in a straightforward way (for details see Appendix 1).
This is not entirely true: The energy dependence of stopping power
functions also slightly depends on the nuclear charge of the sample
material and this effect is more pronounced for light elements
and lower energy, leading to a distortion in the shape of the
spectra in their low energy part, the amount of which depends
on the concentration of light elements in the sample. Although
these effects play a minor role, they become noticeable in high
accuracy analysis and must be considered, when choosing library
standards. Ideally, these standards should have a composition,
very similar to the measured sample. In this case matrix effects
would cancel out.
Fortunately, the samples we are interested in - rocks and soil
- consist to a large part of oxides and oxygen is the predominant
light element that needs to be considered. Also, the concentration
of oxygen in these samples lies generally between 30 and 50 %.
Thus, choosing oxides (with similar concentrations of oxygen)
for the establishment of a standard library, matrix effects due
to the presence of oxygen will to a large extent be taken care
of.
In the case of X-ray analysis, two more matrix dependent effects
need to be considered, i.e. the absorption of X-rays in the matrix
and the effect of secondary emission (for details see Appendix
2). A third order effect - enhancement of emission -can generally
be neglected in geological sample material, although it can play
an important role in the analysis of e.g. metal alloys. The same
approach - using oxide standards rather than pure element standards
- will again help to improve the accuracy of analytical results.
7.2.2 Least Squares Analysis and Programs
The program ALPHA.EXE performs a weighted least squares analysis
of a given set of spectra - the "Library" - on
an "Unknown" spectrum. input data are read from
binary data pools, output contains the %-fractions of the individual
library components t hat yield "best fit", an
estimate of the error and a reduced 2. Output is written to a
file ALPHA.LIS, a second file ALPHA.DAT contains the original
and fitted data and the residuum and can be used for plotting
of fit and residuum.
The present version also contains an option for matrix correction
as applied to alpha backscatter spectra: Results of the least
squares analysis are multiplied with a matrix factor (manual input)
and normalized to 100 % .
The algorithm assumes that only the unknown spectrum contains
errors and that the library spectra are error-free. The errors
determine the weight function; three options are available:
1. Weight = 1/2
2. Weight = 1 (equal weight for all data points)
3. Weight input from an external file
Prior to analysis a normalization of library and unknown is performed:
The content of the first channel of the unknown and the libraries
is read (usually this channel contains the measuring time of the
spectrum) and all libraries are scaled by the ratio of library/unknown
in this channel.
The following constraints are possible:
1. Library component "free"
2. Library component "known"
3. Library component "known relative" to other library
components ("group")
In case 2. the library component is first multiplied with the
"known" concentration and then subtracted from the unknown.
This component is further not considered in the least squares
computation. In case 3. library components of a "group"
are multiplied with their relative "known" concentration
and added. The scaled sum of these components is then used as
a combined library spectrum, i.e. these components are fitted
in a constant ratio to one another.
The program further permits the choice of the range, over which
fits are computed.
7.2.3 X-ray Analysis Software
The analysis of the X-ray data will be conveniently split into
two parts:
(1) the determination of the X-ray intensities and
(2) the determination of the elemental amounts from the X-ray
intensities.
The interpretation of the X-ray spectra to obtain the first order
of qualitative elemental analysis of the measured samples is very
straightforward and it is achieved simply by comparing the energies
of the prominent X-ray peaks in the spectra with the characteristic
Ka and Kb X-ray lines of corresponding elements. Since the resolution
of the system is such that it can separate the Ka from Kb lines
for the elements above about S, the presence of Ka lines expects
the accompanying Kb lines in the proper ratio. This increases
the confidence in correct identification of all peaks in the spectrum.
For less prominent peaks, more elaborate fitting procedures will
yield identification of less abundant elements.
There are several commercial and private program available for
qualitative and semi-quantitative analysis. A common spectrum
decomposition problem is to get the following spectral parameters:
a number of total spectral lines and their centroids, intensities
from measured experimental data and their errors. We have been
successfully using our own FORTRAN programs developed few years
ago for the Phobos missions. Recently, for comparison reasons,
we have also started to use AXIL, PIXE and other programs. All
the programs use profiles of lines of pure chemical elements,
background suppression, and least-squares fit analysis to get
the peak intensities.
After the X-ray peaks and their intensities have been identified
and determined, interelement effects between the analyzed element
and neighboring elements must be corrected for in order to obtain
the elemental abundances. There basically two main methods for
such corrections:
(1) empirical correction procedure
(2) model calculations based on fundamental parameters
The empirical correction method requires measurements of a number
of standard samples of known composition to determine the correction
coefficients. Comparing the measured intensities from the sample
for each element, and from the standard, yield the compositional
information. The disadvantage of this method is a need for a standard
of known composition that is close to the composition of the unknown.
The model calculations, employing fundamental parameters, on the
other hand does not require standard samples. It attempts to model
the physics of X-ray production and absorption in the examined
sample as an effective means for interelement effect corrections.
The authors wish to acknowledge the help during the entire period
of the project from the personnel of the following institutions:
. J. Brückner, J. Huth, H. Kruse of the Max-Planck-Institut
für Chemie in Mainz; M. Perkins, F. DiDonna, L. DiDonna,
F. Sopron, T. Tuzzolino, E. LaRue, J. Barnes of the University
of Chicago; J. Wellman, T. Tomey, R. Bloomquist, H. Kubo, J. Crisp
and the entire rover team at the Jet Propulsion Laboratory; S.
Ryadchenko of the Research Institute for Atomic Reactors in Dimitrovgrad,
Russia; B. Andreichikov, B. Korchuganov and I. Akhmetchin of the
Space Research Institute of the Russian Academy of Sciences.
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Figure 1. The scattering energy of the alpha particles
from an element of mass A is a
function of scattering angle and the mass A. In general,
the alpha particles will
achieve the highest energies when scattered form the heaviest
element
Figure 2. The alpha backscattering has its best resolution
to separate individual neighboring
elements at low Z. For the higher Z elements, only groups
of elements can be
determined. The X-ray fluorescence, however, has its best
resolution for high Z
elements. The combination of both techniques, together
with the proton mode, results
in the ability of the APXS to resolve and determine all
elements, except hydrogen.
Figure 3. The APXS flight instrument for the Pathfinder
mission consisting of the sensor head
and the main electronics.
Figure 4. Composite view of the APXS sensor head showing
the geometrical position of the
main components.
Figure 5. Photograph of the APXS sensor head and the deployment
mechanism mounted on
the back of the rover.
Figure 6. The APXS can correctly distinguish between protons
originating in the sample from
(a,p) reactions and protons of cosmic ray origin that only
contribute to the
background. Fig. 6a shows the distribution of deposited
energies in detectors D1
and D2 for sample protons, while Fig. 6b shows the same
for the cosmic ray protons.
Figure 7. X-ray sensor head: geometrical position of main
components of the X-ray
spectrometer housing. The top cover has a built-in a tungsten
collimator that also
shields the detector from the 244Cm sources.
Figure 8. Electronics block diagram of the APXS showing
the alpha, proton and X-ray analog
and digital circuitry, the microcontroler and the I/O circuitry
to interface with the
rover.
Figure 9. The APXS sensor head is deployed to the Martian
surface by the APXS Deployment
Mechanism (ADM) that enables it to analyze soil and rock
samples in vertical (Fig.
9a) as well as in horizontal position (Fig. 9b).
Figure 10. Alpha (Fig. 10a) and proton (Fig. 10b) spectra
of an igneous rock obtained with the
APXS instrument during preliminary calibration.
Figure 11. The X-ray spectrum of the Allende meteorite
obtained under identical conditions with
an earlier version of the APXS instrument based on HgI2
(Fig. 11a) and a Si PIN
X-ray detector (Fig. 11b) used currently on the Pathfinder
mission.
